Automatic fidelity control



March 11, 1941. A. w. BARBER 2,234,514

AUTOMATIC FIDELITY CONTROL Filed Dec. 21, 1938 Y v 1 SPEAKER;

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I FORMER l:-o -I-n m-L: --o e---o o- VA R. COUPLING 10 no. 20 no.

FREQ. CONTROL TUN'ED TUNED osc. TUBES FILTER FILTER l Z RECTI REcTI- {9 FIE R FIER l SELECTOR OSCILLATOR FIRST DETECTOR INVENTOR.

SOUND PREssuRE FREQUENCY Patented Mar. 11, 1941 UNITED STATES PATENT OFFICE Claims.

This present invention of mine concerns improvements in radio receivers. It particularly relates to methods of, and means for, the automatic control of radio receiver fidelity as a function of the strength of desired and undesired signals.

One object of my invention is to provide automatic fidelity control in a radio receiver which operates as'a function of the strength of desired and undesired signals. Another object isto provide a band expansion system which expands as a function of the amplitude of the desired signal and contracts as a function of the amplitude of undesired signals. Still another object is to provide fidelity control which expands as a function 'of the strength of the signal being received and contracts as a function of undesired signals especially those in adjacent channels such as those 10 and kilocycles away when operating in 20 the broadcast band. A further object is to provide circuits employing diode rectified tubes in which one diode controls the response expansion as a function of the desired signal and the other diode provides a counteracting contraction as a function of the beat note produced by interfering signals.

A still further object is to provide a fidelity control which depends on the audio frequency gain control setting or the audio output of the receiver however determined.

The invention herein set forth has been disclosed in my copending application entitled Automatic fidelity control filed on April 6, 1936, and bearing Serial No. 72,875 but certain features will be more fully set forth and claimed in this continuation in part.

In general, the stronger a received radio signal, the broader the reception band may be made. Static, for instance, is a continuous spectrum which causes disturbances which are a function of the band width of the receiver response. When the desired signals are weak, static is a serious limitation and the best reception of the desired signal is obtained by narrowing the receiver response. Interfering signals are also to a lesser and more random degreea function of desired signal strength and again narrow band reception for weak signals is a partial solution. It is for these reasons that tone and selectivity con- 50 trol systems have been developed which automatically control the audio frequency response or radio frequency selectivity as a function of received signal strength. These systems are designed to broaden the receiver response when re- :ceiving' signalsof large amplitude. My Patents Nos. 2,120,998 and 2,136,664 issued on June 21, 1938, and November 15, 1938, and entitled Coupled circuits and Coupled circuit systems respectively, show systems whereby the receiver fidelity is automatically expanded as a function of received signal strength. My present system may be used in conjunction with the circuits shown in these patents.

I have found that while signal strength is an important criterion for determining usable receiver response, there are conditions of reception which require additional control. The most important case of interference not adequately provided for by the signal strength type of control I have found to be due to adjacent channel f stations. In this country in the standard broadcast band the stations are positioned 10,000 cycles apart and, adjacent channel interference ischaracterized by a 10,000 cycle beat note. This beat note is the difference frequency between thecarrier of the station being received and the interfering station which may be on either or both sidechannels. Similarly, stations two channels removed produce 20,000 cycle characteristic beat notes. While the interference from these sidechannel stations is due to both their carriers and side-bands the beat notes between desired and undesired station carriers are an indication of the amount of interference and may be used for band width control purposes. I have found that i a control system which increases response width as a function of increased signal strength of the desired station and decreases response width as a function of 10,000 and 20,000 cycle beat notes minimizes interference from static and adjacent channel stations. In carrying out this dual control'system I generate a control voltage proportional to the desired signal and a second but opposed control voltage proportional to the10 and 20:kilocycle beatinotes, These control voltages are applied to response adjusting means so that the'receiver response is broadened by increased desired signal strengths and is narrowed due to 10 and 20 kilocycle beats. The controls may be variously weighted,- delayed controls 'r'n'ay' be used or other beats may be chosen' The channels for some television transmission are 6000 kilocycles apart in which case 6000'kilocycle beats maybe used for control purposes. I prefer to derive thecontrol beat note from'the output'of the audio amplifier'of the receiver tor atl'east from a point beyond the point ofmanualvolume control since in this. way the actual soundilevel is taken into accountjn the band width control.

' The appended. claims set forth,:in particular, v;

the novel features to be found in this invention. The following description, however, when taken in connection with the drawing, will serve to set forth the theory and mode of operation of my invention.

In the drawing,

Fig. 1 shows a block diagram of one form of my invention.

Fig. 2 shows one form of my invention embodied in a radio receiver.

Fig. 3 shows various response curves of the human ear which are useful in explaining the operation of my invention.

In Fig. 1, I have shown a block diagram of one form of my invention. I have shown a superheterodyne radio receiver with coupling control tubes controlled so as to expand the receiver response on strong desired signals and to contract the receiver response in the presence of 10 acteristics of a pair of coupled circuitsor a multistage amplifier may be controlled as a function of received signal amplitude. In order to accomplish this a direct current component of rectification from the second detector is applied to the control grids of the coupling control tubes.

My present invention consists in applying a counteracting control to the coupling control tubes-which is a function of interfering signals. Since the most serious interference will be due to stations in the next channel on either side of the desired carrier, I select the accompanying 10 kilocycle beat note with a tuned filter and rectify the resulting voltages. This rectified voltage is then subtracted from the desired signal generatedcontrol voltage producing a net control voltage which depends on the amplitudes of desired and undesired signals. Similarly the interference from the next removed channels may be taken into account bymeans of a 20 kilocycle tuned filter and associated rectifier the output of which is also subtracted from the normal amplitude generated control voltage. Additional control may be derived from 30 kilocycle beats or higher or other separations may be chosen as for instance 6000 kilocycles which is the channel separation for some television transmissions. In Fig. 1, the 10 kilocycle filter is shown as 10 k. c. tuned filter receiving an input from the audio ampli-, fier output. The filter output is rectified by the rectifier. Similarly the 20 kilocycle filter is connected to the audio amplifier output and feeds a second rectifier. The output of these two rectifiers is subtracted from the amplitude generated control voltage from the 2nd detector by reversing the polarity of the outputs from the beat note rectifiers with respect to the amplitude operated rectifier. Thus strong desired signals which are normally above the static level expand the receiver response but stations 10 or 20 kilocycles removed contract. the response.

In Fig. 2, I have shown a schematic diagram of the essential circuits of a system similar to that of Fig. 1. In order to show the method of connection and operation of the system without undue complication, a single amplifier interstage coupling transformer is shown with automatic coupling control derived from a combination of signal amplitude and two beat frequency interference components as for instance 10 and 20 kilocycles. This is not intended to in any way limit the system to a single stage as several stages may be controlled and many combinations of control are possible.

The system in Fig. 2 shows a pre-selector, oscillator and first detector in block connected to an antenna A and a ground G. This unit may also include one or more controlled coupling transformers and amplifier stages. The output of the unit feeds a primary coil l tuned by a condenser 2. An intermediate frequency amplifier tube 3' receives on its grid 4 the voltage across a secondary coil 5 tuned by condenser 6. The coupling between primary I and secondaryv 5 is accomplished by the link coils l and- 8 in series, in which coil I is coupled to primary l and coil 8 is coupled to secondary 5. This coupling link circuit is completed between ground G back to ground G again thru the blocking condenser 9 and the input capacity of the vacuum tube III. The object of this invention is to provide an improved control system in connection with this coupling link or other types of expansion control.

The output system associated with amplifier tube 3 comprises a primary coil I I tuned by condenser i2 and connected between a source of plate voltage and plate l3 of tube 3. Coupled to primary II are two secondary coils l4 and I5. While tuning is not shown, these secondaries -l4 and I5 may be tuned in conventional ways. Secondary I4 is connected between ground G and anode l6 of the double diode tube II. The cathode l8 corresponding to anode l6 and forming a diode rectifier therewith is connected to ground G thru the load resistor 20 by-passed by condenser 2I. When a signal traverses the system, a rectified voltage drop appears across resistor 20 having an amplitude proportional to the signal. Secondary I5 is connected at one end to anode 22 of. the second diode unit of rectifier I1 and at the, other end to ground G. thrua load resistor 24 by-passed by condenser 25. The second diode circuit is completed by ground cathode 23. The time constant of condenser 25 and resistor 24 is small and suitable for generating an audiofrequency demodulation drop-when a modulatedsignal is passed thru the system. The input to the audio amplifier tube 60' consists of a-portion of the drop across resistor 24 as determined by the setting of tap 6| on volume control potentiometer 59 coupled to resistor 24 thru condenser 58. The drop across resistor 24 may also-be used for automatic volume control byfiltering out the audio frequency components by means of series resistor 26 and by-pass condenser 21-. The filtered voltage drop across resistor 24 is.applied to the grid 4 of amplifier tube. 3 thru secondary 5. The polarity is such that the. coil end of resistor 24 is. negative with respect to. ground which gives the desired negative. voltage for automatic volume control on an amplifierwtube grid.

The rectified drop across resistor 20will 'yield a voltage which. is. a function of the strength .of the desired signal. The. current will flow. in such a direction as to make the cathode end afresistor 20. positive with respect to ground G. This positive voltage is applied thru resistor- 28 by passed by condenser 29, resistor 30 by-passed by condenser 3| to grid 32 of coupling control tube I The input or grid to cathode capacity of tube a function of the total plate to grid capacity 34, the plate load impedance 35 which maybe a simple resistor or a complex impedance, the net efiective grid voltage and the characteristics of the tube. Plate 38 is energized'by a steadyvoltage and cathode 39 is initially biased by a by-passed cathode resistor or by other well known means. The input capacity of tube [0 will increase as the net negative bias on grid 32 is reduced as occurs when increasing desired signals are received and rectified by diode H. An increase in input capacity of tube [0 decreases the series impedance of the coupling link circuit between coils I and 5. Hence, the system as thus far described operates to increase the coupling between coils l and 5 as the desired signal increases. If the coupling link is set up to give critical coupling in the absence of a received signal, the coupling will be more than critical for appreciable values of received signal and the 'interstage transformer (coils l and 5 and associated circuits) will show a band-pass characteristic of a width which increases as the strength of the desired signal increases,

Signals in adjacent channels beating with the desired signals set up beat notes which are utilized to contract the intermediate amplifier response or to counteract the expansion caused by the desired signal.

rier is demodulated and beat notes between de- 35* The modulated desired carsired and'undesired signals are produced in the dioderectifier comprising plate 22 and cathode 23 in Fig. 2. This rectifier is fed with interme' diate frequency signals from the intermediate frequency amplifier by means of input coil l5 coupledto coil II and feeding plate 22 directly and cathode 23 thru the modulation frequency load consisting of resistor 24 by-passed by intermediate frequency by-pass condenser 25. Modulation frequency voltages and beat notes produced across resistor 24 are fed to volume control potentiometer 59 thru coupling condenser 58. A first audio frequency amplifier stage consisting of amplifier tube 60' with plate 68, control grid 6! and cathode B2 is fed from potentiometer 59 by connecting potentiometer variable tap point 6! to control grid 6|. Tube 60' feeds primary 63 of push-pull input transformer 63B4 and secondary 54 feeds grids 61 and 10 of pushpull amplifier tubes 65 and 65'. These push-pull tubes have cathodes 58 and H and plates 66 and 69 respectively. Cathodes 68 and H are shunted by center tapped resistor 12 and receive a bias from cathode current flowing in resistor 13. Plates 66 and. 69 feed primary 'l415 of a speaker coupling transformer feeding speaker S.

Audio frequency voltages and beat notes appear after volume control and amplification across primary 'l4l5 of the speaker coupling transformer. The beat notes are selected from primary l4 and after rectification are used to control fidelity. The coil 44 is tuned to 10 kilocycles by condenser 43 and the selector circuit thus formed is coupled to primary 14 thru coupling condenser 42. The 10 kilocycle beat notes thus selected are further selected by coil 45 tuned to 10 kilocycles by condenser 45 and magnetically coupled to coil 44. The 10 kilocycle beat notes thus selected are applied to rectifier 41 comprising diode plate 48 and cathode 49 thru a rectification current load comprising resistor 28 by-pas'sed by condenser 29. Thus '10 kilocycle beat notes appearing in the system are rectified by tube 41 and produce a direct current voltage across resistor 28 in which the coil end of resistor 28 is made negative with respect to the cathode end. This drop is applied in series with other voltages applied to the control grid of tube ID in a sense such that the gain of tube It is decreased by the presence of 10 kilocycle beat notes and hence its effective input capacity, the coupling between coils I and 5, and the band passed by the intermediate frequency amplifier are all decreased.

Similarly coil 52 is tuned to 20 kilocycles by condenser 5! and the 210 kilocycle selector thus formed is coupled to primary 14 thru coupling condenser 59. Further 20 kilocycle selection is provided by coil 53 tuned to 20 kilocycles by condenser 54 and coupled magnetically to coil 52. The selected 20 kilocyclebeat notes are applied to plate 55 and cathode 51 of rectifier tube 55 thru rectification products. load comprising resistor 36 by-passed by condenser 3|. Thus 20 kilocycle beat notes appearingin the system produce a direct current voltage. drop across resistor 30 and coil end of resistor 36 is made negative with respect to cathode 51. The voltage drop across resistor 35] is added to the other voltages on the control grid of tube iii in a sense which makes this grid more negative or less positive due to the presence of 20. kilocycle beat notes received by the system. Thus 20 kilocycle beat notes decrease the gain-of tube II), the coupling between coils I and 5 and the width. of the frequency band passed by the intermediate frequency amplifier.

It will be seen that the audio volume control 59 is included in the system ahead of the point at which the beat notes are taken off and ape plied to the rectifiers and fidelity control tube. In Fig. 3 is shown a set of audibility curves showing plots of sound pressure against frequency. Curve A is for barely audible sounds,

curve B is for intermediate sound levels' and curve C is for sounds so loud that they are felt as they are heard. Curves A, B and C are curves of apparent equal loudness of sound. Thus sounds of low levels intensity require considerably higher relative intensities at high frequencies to sound as loud as sounds of medium frequency. As the level of sound is increased the intensities of high frequency sounds becomes relatively less to sound of the same loudness as middle frequency sounds and finally at very high levels the curve is actually reversed.

Now, a system such as the one which I have described may be adjusted so that the contraction of band-width due to beat notes bears a certain relation to the expansion of band-width due to the desired carrier. This relation may be determined at low sound levels from the loud speaker so that the interference is not audible. As the sound level is increased by advancing volume control 596l the effectiveness of the beat notes in contracting the receiver response is increased over the expansion due to the desired signal carrier due to the fact that the beat notes are amplified by a greater relative amount than for the initial condition. This greater relative contraction due to beat notes is necessary since at the higher levels the high frequency region containing most of the interfering sounds becomes relatively more audible. The system thus operates to produce asubstantially equal apparent interference ratio at all usual output sound levels. The primary requirement for this equality of apparent interference is that'the interference control (beat notes) be amplified proportional to the sound output. It should be noted that a volume control placed before the point at which the expansion control voltage is taken off will not produce this desired effect since the relative effectiveness of the expansion and contraction controls is not then relatively variable.

While I have shown and described only one system whereby my invention may be carried into efiect many modifications are possible within the spirit and scope of my invention as set forth in the appended claims.

What I claim is:

1. In a carrier wave receiver, the combination of, a selective carrier wave amplifier, means for varying the selectivity of said carrier wave amplifier, a detector, a modulation frequency amplifier, a gain control device connected between said detector and said modulation frequency amplifier, a loud speaker connected to the output of said modulation frequency amplifier, and means for deriving a control signal from said modulation frequency amplifier for at least partially controlling said selectivity varying means whereby varying the input to said loud speaker by means of said gain control in the presence of a control signal varies the selectivity of said carrier wave amplifier.

2. In a modulated carrier wave receiver, the combination of a variably selective circuit, means for receiving a desired modulated carrier wave and undesired signals, means for varying the selectivity of said circuit, means for deriving from said desired signal a signal to at least partially control said selectivity varying means, means for producing modulation frequency signals from said modulated carrier Wave and beat note signals from said desired and undesired signals, means for simultaneously controlling the amplitude of said modulation frequency signals and said beat note signals after said production, and

means for deriving an additional signal from said beat note signals to at least partially control said selectivity varying means.

3. In a modulated carrier wave receiver, the combination of a variably selective modulated carrier wave amplifier, means for receiving a desired modulated carrier wave and undesired signals, means for varying the selectivity of said carrier wave amplifier, means for deriving a signal to control said selectivity varying means from said desired signal, means for producing modulation frequency signals from said modulated carrier and beat note signals between said desired carrier and said undesired signals, means for slmultaneously varying the amplitude of said mod- I ulation frequency signals and said heat notes after said production, and means for deriving an additional signal to control said selectivity varying means from said beat notes.

4. In a carrier wave receiver the combination of, a selective circuit, means for controlling the selectivity of said circuit, a modulated carrier signal amplifier including an output circuit, a rectifier connected between said output circuit and said selectivity control means, a modulation signal amplifier including a gain control and an output circuit for feeding a loud speaker, and a second rectifier connected between said second output circuit and said selectivity control means.

5. The method of controlling the selectivity of a modulated carrier wave receiver including a demodulator and modulation frequency gain control means from desired signals and beat notes derived from desired and undesired signals which comprises, varying the selectivity of said receiver in accordance with the amplitude of desired signals in a predetermined sense, simultaneously varying the gain applied to modulation frequency signals and beat notes after demodulation, and varying the selectivity of said receiver in accordance with the amplitude of the resulting beat notes in the opposite sense.

ALFRED W. BARBER. 

